Procaryotic viruses have been relatively studied and used as cloning vectors for the past decade. Unfortunately, procaryotes do not provide sufficient cellular mechanisms for the expression of many gene products of viral interest for eukaroytic cells.
A readily identifiable phenotype is of primary concern for identifying a cloned gene. Thus, a vector system is required that allows expression of the cloned sequence.
Furthermore, cloning a specific gene requires the purification and amplification of that specific entity from an environment of many genes. One powerful purification and amplification lies in "rescue." By this is meant that the gene of interest can be isolated from a background of many genes by incorporation into an infectious virus that can be purified by standard microbial techniques. The vector is linked to the fragmented genomic DNA and introduced into cells that can express the desired phenotype.
After several rounds of replication, the expressed genome, the genome of interest, is an abundant product. The genome, appropriately tested, will show that phenotype that characterizes it. This screening technique cannot be used in bacterial environments which lack functions of eukaryotic cells.